The Math Meridian
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The sine law

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Learn it In any triangle, a/sin A = b/sin B = c/sin C (each side over the sine of the angle opposite it). Use it when you know a side and its opposite angle, plus one more side or angle.

Example: In △ABC, ∠A = 40°, ∠B = 65° and a = 12 cm. Find b.

  1. b/sin 65° = 12/sin 40°.
  2. b = 12 sin 65° ÷ sin 40° ≈ 16.9 cm.

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